The present invention relates to nominal ZrB.sub.2 powders and shapes having lower electrical resistivity, higher thermal conductivity, and improved thermal shock resistance over previously reported ZrB.sub.2 phase materials.
The compound ZrB.sub.2 is a particularly important ceramic because it exhibits near isotropic thermal expansion, a very desirable characteristic for a ceramic used at elevated temperatures and/or used in thermal cycling operations. Remarkably, ZrB.sub.2 displays these properties even though it has a hexagonal crystal structure that is made up of alternating layers of Zr and B atoms. The electrical resistivity of ZrB.sub.2 is also near isotropic and the value obtained on single crystals is approximately 3 microhm-cm which is quite low and similar to those of Al metal. ZrB.sub.2 appears to have the lowest electrical resistivity of any known nonmetallic compound, other than the intercalated graphites which are rather unstable above room temperature.
Although single-crystal ZrB.sub.2 exhibits a low electrical resistivity, fabrication of ZrB.sub.2 shapes by conventional hot pressing with known or reproducible electrical properties has proven difficult to achieve to date. Further, it has been difficult to obtain electrical resistivities on powder metallurgy-prepared material within a factor of approximately two (2) of single-crystal ZrB.sub.2. Also, large variations in electrical resistivity are observed with ZrB.sub.2 powders purchased from various vendors, as well as among batch-to-batch variations from individual vendors. It is not unusual to find electrical resistivity values stated in the literature that vary by factors of three or more.
Many researchers have processed ZrB.sub.2 powders into various shapes, but none have attempted to lower the electrical resistivity of the samples they fabricated. For example, U.S. Pat. No. 4,929,417 describes efforts to fabricate ZrB.sub.2 at lower temperatures such as by adding both elemental Zr and B to the ZrB.sub.2. The elemental Zr and B are added in the appropriate ratio to form more ZrB.sub.2 and reportedly react during hot pressing to produce a dense homogeneous and single-phase ZrB.sub.2 body. U.S. Pat. No. 3,937,619 reports adding Ti, Zr, or Hf to ZrB.sub.2 to form ternary monoborides such as (Ti,Zr)B in equilibrium with the ZrB.sub.2. These workers report these monoborides give a hard, tough, and strong ceramic body. Several other references, e.g., U.S. Pat. Nos. 4,419,161; 4,636,481; and 4,923,829, report formation of sintered ceramic materials using ZrB.sub.2. None of these references report lowering the electrical resistivity of ZrB.sub.2 to levels approaching those of single-crystal ZrB.sub.2-x.